I've had that happen in EMDR. My therapist saw it start and pulled me out. Can't remember how. Grounding of some kind. After that it was just looping around the same image. Later on I met that part, and I'm really glad I didn't jump into those memories, it was something I'm still not ready for.
I had another part introduce himself to me and tell me he was going to interrupt session if we did EMDR. He intended to come up front and talk to my therapist if I did, and with his character and my therapist's experience (this how I found out about my DID) that idea was a bit horrifying. He told me he was onboard with therapy but I couldn't handle the memories that would come up next. I listened to him